1. Field of the Invention
The present invention relates to a receiver, and more particularly to a receiver for use in a CDMA (Code Division Multiplex Access) communication system.
2. Description of the Related Art
Receivers for use in broadband CDMA communication systems which are expected to be next-generation portable telephone system standards should comprise RAKE receivers for removing multipath fading interference.
In a CDMA communication system, as shown in FIG. 1 of the accompanying drawings, multiplier 102 in transmitter 101 spreads transmission data x(t) by multiplying it by spreading code c(t), and the transmitter 101 transmits the spread data as transmission signal s(t).
The transmission signal s(t) transmitted from the transmitter 101 is subjected to multipath fading over a plurality of transmission paths 103-1 through 103-N with respective delays τ1-τN. Thereafter, the signal is received as a reception signal r(t) by a RAKE receiver 104.
Specifically, over the respective transmission paths 103-1-103-N, the transmission signal s(t) is given the respective delays τ1-τN, and the results are multiplied by respective coefficients a1-aN representing respective phase/amplitude ratios of the transmission paths. The signals from the transmission paths 103-1 through 103-N are then added together, producing the reception signal r(t).
As shown in FIG. 2 of the accompanying drawings, the RAKE receiver 104 has a plurality of fingers comprising a plurality of delay units 110-1-110-N for giving delays τ1-τN depending on the respective transmission paths to the reception signal r(t), a plurality of multipliers 120-1-120-N for multiplying signals which are produced by delaying the reception signal r(t) for the delays τ1-τN with the delay units 110-1-110-N, by a complex conjugate value c(t)* of c(t), and outputting product signals, a plurality of integrators 130-1-130-N for integrating the product signals from the multipliers 120-1-120-N for the period of one symbol, and outputting integrals f1,n-fN,n, respectively, a plurality of multipliers 140-1-140-N for multiplying the integrals f1,n-fN,n outputted from the integrators 130-1-130-N by complex conjugate values a1*-aN* of the respective coefficients a1-aN, and outputting product signals, and an adder 150 for adding the product signals from the multipliers 140-1-140-N and outputting the sum as an output signal series Rn. The reception signal r(t) should ideally be branched into as many as signals as the number of transmission paths, but may not be so branched because of limitations on the circuit scale.
Multiplying the integrals f1,n-fN,n outputted from the integrators 130-1-130-N by the complex conjugate values a1*-aN* with the multipliers 140-1-140-N is equivalent to correcting carrier phase differences caused between the transmission paths over the transmission paths and weighting the signals depending on amplitude differences between the transmission paths. Therefore, when the signals outputted from the multipliers 140-1-140-N are added by the adder 150, their vectors can be combined for a maximum S/N ratio.
A process of transmitting and receiving data in the above CDMA communication system will be described below in specific detail with reference to FIGS. 1 and 2.
The transmission signal s(t) transmitted from the transmitter is produced by multiplying the transmission data x(t) by the spreading code c(t), as shown by the following equation (1):S(t)=c(t)×(t)  (1) 
The transmission data x(t) is represented by a signal where the value of a transmission data series xn continues for a symbol interval T, as shown by the following equation (2):x(t)=xn . . . nT≦t<(n+1)T  (2) 
The reception signal r(t) which has been subjected to multipath fading is produced by giving the respective delays τi of the transmission paths to the transmission signal s(t), multiplying the results by the respective coefficients ai representing phase/amplitude ratios of the respective transmission paths, and adding the signals from all the transmission paths, producing the reception signal r(t), as shown by the following equation (3):                               r          ⁢                                           ⁢                      (            t            )                          =                                            ∑                              i                =                1                            N                        ⁢                                                   ⁢                                          a                i                            ⁢                                                           ⁢              s              ⁢                                                           ⁢                              (                                  t                  -                                      τ                    i                                                  )                                              =                                    ∑                              i                =                1                            N                        ⁢                                                   ⁢                                          a                1                            ⁢              c              ⁢                                                           ⁢                              (                                  t                  -                                      τ                    i                                                  )                            ⁢                                                           ⁢              x              ⁢                                                           ⁢                              (                                  t                  -                                      τ                    i                                                  )                                                                        (        3        )            
In the fingers of the receiver, the delay units 110-1-110-N give the reception signal r(t) the delay times τi of the transmission paths, as shown by the equation (4) below. Then, the multipliers 120-1-120-N multiplies the signals from the delay units 110-1-110-N by the complex conjugate value c(t)* of the spreading code. Thereafter, the integrators 130-1-130-N integrate the product signals from the multipliers 120-1-120-N for the period of one symbol, producing integrals fj,n. As shown by the equation (5) below, the integrals fj,n outputted from the integrators 130-1-130-N are represented by the sum of the products of the transmission data series xn and the phase/amplitude ratios of the transmission paths, and interferences Ij,n composed of signal components from other transmission paths having different delay times.                                                                         f                jn                            =                            ⁢                                                                    ∫                    nT                                                                  (                                                  n                          +                          1                                                )                                            ⁢                                                                                           ⁢                      T                                                        ⁢                                      r                    ⁢                                                                                   ⁢                                          (                                              t                        +                                                  τ                          j                                                                    )                                        ⁢                                                                                   ⁢                    c                    *                                          (                      t                      )                                        ⁢                                                                                   ⁢                                          ⅆ                      t                                                                      =                                                      ∫                    nT                                                                  (                                                  n                          +                          1                                                )                                            ⁢                                                                                           ⁢                      T                                                        ⁢                                                            ∑                                              i                        =                        1                                            N                                        ⁢                                                                                   ⁢                                                                  a                        i                                            ⁢                                                                                           ⁢                      c                      ⁢                                                                                           ⁢                                              (                                                  t                          -                                                      τ                            i                                                    +                                                                                                                                                                                                                            ⁢                                  τ                  j                                )                            ⁢                                                           ⁢              x              ⁢                                                           ⁢                              (                                  t                  -                                      τ                    i                                    +                                      τ                    j                                                  )                            ⁢                                                           ⁢              c              *                              (                t                )                            ⁢                              ⅆ                t                                                                        (        4        )                                                                                                                 ⁢                              =                                ⁢                                                                            a                      j                                        ⁢                                                                                   ⁢                                                                  ∫                        nT                                                                              (                                                          n                              +                              1                                                        )                                                    ⁢                                                                                                           ⁢                          T                                                                    ⁢                                              x                        ⁢                                                                                                   ⁢                                                  (                          t                          )                                                ⁢                                                                                                   ⁢                                                  ⅆ                          t                                                                                                      +                                                            ∑                                              i                        =                        1                                                                    i                        ≠                        j                                                              ⁢                                                                                   ⁢                                                                  a                        i                                            ⁢                                                                                           ⁢                                                                        ∫                          nT                                                                                    (                                                              n                                +                                1                                                            )                                                        ⁢                                                                                                                   ⁢                            T                                                                          ⁢                                                  c                          *                                                      (                            t                            )                                                    ⁢                                                                                                           ⁢                          c                          ⁢                                                                                                           ⁢                                                      (                                                          t                              -                                                              τ                                i                                                            +                                                                                                                                                                                                                                                                                  ⁢                                  τ                  j                                )                            ⁢              x              ⁢                                                           ⁢                              (                                  t                  -                                      τ                    i                                    +                                      τ                    j                                                  )                            ⁢                                                           ⁢                              ⅆ                t                                                                                        =                            ⁢                                                                    a                    j                                    ⁢                                                                           ⁢                                      x                    n                                                  +                                  I                  jn                                                                                        (        5        )            
Then, the multipliers 140-1-140-N multiply the integrals fj,n outputted from the integrators 130-1-130-N by the complex conjugate values aj* of the phase/amplitude ratios of the transmission paths. Thereafter, the adder 150 adds the outputs from the fingers together, and outputs the sum as the output signal series Rn, as shown by the equations (6), (7) below. When the multipliers 140-1-140-N multiply the integrals fj,n by the complex conjugate values aj* of the phase/amplitude ratios, the phase errors of the respective paths are corrected, and the signals are weighted for a maximum S/N ratio.                     Rn        =                                            ∑                              i                =                1                            N                        ⁢                                                   ⁢                                          a                i                            *                              f                jn                                              =                                                    ∑                                  j                  =                  1                                N                            ⁢                                                           ⁢                                                                                                              a                      j                                                                            2                                ⁢                                                                   ⁢                                  x                  n                                                      +                                          ∑                                  j                  =                  1                                N                            ⁢                                                           ⁢                                                a                  j                                *                                  I                  jn                                                                                        (        6        )                                                           ⁢                  =                                                    x                n                            ⁢                                                           ⁢                                                ∑                                      j                    =                    1                                    N                                ⁢                                                                   ⁢                                                                                                a                      j                                                                            2                                                      +                                          ∑                                  j                  =                  1                                N                            ⁢                                                           ⁢                                                a                  j                                *                                  I                  jn                                                                                        (        7        )            
The RAKE receiver tends to have a large circuit scale because it needs to have a plurality of despreaders parallel to each other for despreading the reception signal.
However, terminals which have the RAKE receiver are required to have a small circuit scale in view of demands for a low price and power requirements.
If terminals have a large circuit scale due to the incorporation of the RAKE receiver, then the price and power requirements of the terminals cannot be reduced.